To sign in= you sign yourself into the visitors book

To be signed into = someone authorizes you to be signed in. and would counter sign you as being their visitor.

Examples. A classroom. you sign in is when you just sign in the attendance book

To be signed in, The teacher would have you sign in and then he would sign off that you were there.

In a factory. To be signed in, You would meet someone at the reception desk and they would sign in that they are responsible for you and you would also sign the Visitors book.

To sign in, you just go to the reception and sign the visitors book and walk on through.
If you were stopped you could be asked "have you been signed in?" you would say "I signed the visitors book" they would say "Yes but, who signed you in?"

I hope this is clear.

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Note added at 4 hrs (2008-11-25 02:37:37 GMT)
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Also. You can be signed into an agreement, Legal document.

Being signed into an agreement is where you signed an agreement and you have to abide by that agreement. Ie; locked into it, Signed into a contract,

You can also use Signed into like this .
"I have signed into the meeting as the representative for NATO, Not as my usual role as adviser to the Government." You would do this if you had several areas of responsibility.

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Note added at 5 days (2008-11-29 22:50:22 GMT) Post-grading
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Thank You